The present invention relates to non-aqueous secondary lithium-ion electrochemical cells and batteries.
Lithium-ion batteries are considered to be the rechargeable batteries of the future for portable electronics to aerospace to vehicular applications. In a known construction for a lithium-ion battery, carbon or graphite is used as an anode, a lithiated transition metal intercalation compound is used as a cathode and LiPF6 is used as an electrolyte in carbonate-based nonaqueous solvents.
The electrochemical process is the uptake of lithium ions at the anode during charge and their release during discharge, rather than lithium plating and stripping as occurs in metallic lithium rechargeable battery systems. As metallic lithium is not present in the cell, lithium-ion cells have enhanced safety and a longer cycle life than the cells containing metallic lithium.
At present, disordered carbon (hard carbon) and ordered carbon (graphite) are used as anodes in commercial lithium-ion batteries. The carbonaceous materials can deliver a reversible specific capacity of 372 mAh/g, corresponding to the chemical formula LiC6 as compared to 3830 mAh/g for metallic lithium. The practical reversible capacity of these carbonaceous materials is even lower in the range of 300-340 mAh/g.
Other carbonaceous materials, also of disordered structure and are known as xe2x80x9csoft carbonxe2x80x9d, of high reversible capacity have been prepared by pyrolysis of suitable starting materials. Sato et al (Science, 264, 556, 1994) disclosed a carbonaceous material prepared by heating polyparaphenylene at 700xc2x0 C. which has a reversible capacity of 680 mAh/g. Mabuchi et al (Seventh International Meeting on Lithium Batteries, Extended Abstracts, Page 212, Boston, Mass., 1994) disclosed a low density carbonaceous material prepared by heating coal tar pitch at 700xc2x0 C. which has a reversible capacity of about 750 mAh/g. Yamada et al (U.S. Pat. No. 5,834,138, Nov. 10, 1998) disclosed a carbonaceous material prepared by heat treatment of coffee beans, tea leaves, corns, etc. at 1100-1200xc2x0 C. The carbonaceous material delivers a reversible capacity of 500 mAh/g.
These values of reversible capacities are much greater than that of the carbonaceous materials used in commercial lithium-ion cells. However, low density and very high irreversible capacity loss of the above carbonaceous materials limit their commercial use as anodes for lithium-ion batteries.
It has been suggested that the reversible capacity of anodes formed of carbonaceous materials can be increased by the addition of other elements to the carbonaceous materials. For example, the addition of small amounts of phosphorous (European Patent Application No. EP 486950) and boron (Japanese Application Laid-Open No. 03-245458) are alleged to enhance the specific capacity of a carbonaceous anode. Moreover, Canadian Application Serial No. 2,098,248 discloses that substituting electron acceptors (such as boron, aluminum, etc.) for carbon atoms in the structure of the carbonaceous materials will enhance anode capacity.
The present invention provides a new and different concept for enhancing the reversible capacity of the carbonaceous material forming the active material of an anode in a lithium ion cell or battery. Specifically, the present invention provides a lithium-ion cell in which molybdenum carbide is combined with the carbonaceous material of the anode to enhance the reversible capacity of the carbonaceous material. This concept is also believed to promote the development of high specific energy and energy density lithium-ion cells and batteries.
Accordingly, it is the principal objective of the present invention to improve the reversible capacity of carbonaceous material forming the active material of an anode of a lithium-ion cell or battery.
Another objective of the present invention is to provide a novel and improved rechargeable lithium-ion cell and/or battery having high specific energy and energy density.
Further features of the present invention will become apparent from the following detailed description and the accompanying drawings.